feat(sync): add cleanup loops and metrics for rejected events index

Add automatic cleanup and Prometheus metrics for the two-tier rejected
events index that caches rejected announcements for re-processing.

Cleanup loops:
- Hot cache: Every 60 seconds (events expire after 2 minutes)
- Cold index: Every 24 hours (metadata expires after 7 days)
- Background task with graceful shutdown support

New Prometheus metrics (7):
- Gauges: hot_cache_current, cold_index_current
- Counters: hits, misses, hot_expired, cold_expired, invalidated

This completes the maintainer announcement re-processing feature,
reducing wait time from 24 hours to <1 second when a maintainer's
announcement arrives before the repository owner's announcement.

Memory is bounded through automatic cleanup, and comprehensive metrics
enable monitoring of hit rates, memory usage, and cleanup effectiveness.

Changes:
- src/sync/metrics.rs: Added 7 metrics with recording methods
- src/sync/rejected_index.rs: Added optional metrics support
- src/sync/mod.rs: Added cleanup background task

Tests: 248 library tests passing, 3 integration tests passing

0 files changed, 0 insertions, 0 deletions